Look at any table at Sing Heung Yuen and you'll see diners slurping the same thing. Long queues form at lunchtime for the dai pai dong's macaroni or instant noodles in tomato broth.

The same goes for the crispy buns (right). The dinner rolls are sliced in half, toasted, then topped with lemon marmalade, peanut butter, condensed milk, or any combination of the three.

Irene Li Oi-lin, a second-generation operator of the restaurant, is surprised by how quickly the tomato soup gained a following. "We only introduced this in about 2000," she says.

Although its most famous dishes are relatively new offerings, Sing Hueng Yuen is not. The dai pai dong's licence dates back to 1957, when it started on Elgin Street.

In 1975, it moved to its current spot at the corner of Gough Street, initially sharing the space with beef brisket specialist Kau Kee, which is now directly across the street.

The Lis bought the business from the original owner in the '60s, and the entire family made an effort to make it thrive.

"I started working here when I was 15," says Li, the second of five children. "We used to run the stall together."

Today, three of the siblings run the dai pai dong. The famous tomato broth's recipe was largely developed by Li, although the idea for it came by accident.

"We used to serve food like every other cha chaan teng, but one day, one of our regulars came and he was feeling sick. He asked us to put a couple of slices of tomato in his macaroni soup, saying the acidity might help bring back his appetite."

Word spread and as more people started requesting the dish, Li decided to perfect the soup. Using three different canned tomatoes and fresh ones, the result is a thick, intensely red broth. The noodles are cooked in different flavoured broths before tomato soup is added.

"We alter the proportions of the different canned tomatoes because [their] quality and sweetness change according to the season," says Li. "I taste the soup and adjust it every time. I can't tell you how; it's second nature to me now."

Such efforts have paid off, as customers keep coming back. "Our oldest regular has been coming for 30 years," she says. "Sometimes we know someone's daughter is dating even before her parents do, because the date was here!"

However, there is some uncertainty regarding the stall's future. Dai pai dong licences could only be passed on to a spouse. An exception was made in 2009, when several licences in Central - Sing Heung Yuen among them - were permitted to pass ownership down one generation.

"We didn't think we'd be allowed to stay," says Li, although she is unsure of what will become of the stall in the future. "I don't think there's any point in thinking about the future when it's not in your control. We live for the moment."